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Aug. 1, 2011
Tokyo Institute of Technology, Japan
Toyota Motor Corporation, Japan
High Energy Accelerator Research Organization, Japan
Japan Proton Accelerator Research Complex (J-PARC) Center
Key points of this announcement
Professor Ryoji Kanno and Lecturer Masaaki Hirayama of the Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology and colleagues from Tokyo Institute of Technology, Toyota Motor Corporation, and the High Energy Accelerator Research Organization have discovered a superionic conductor*1 exhibiting the highest lithium-ion conductivity reported to date. The lithium-ion conductivity of this superionic conductor (Li10GeP2S12) was found to be 12 mS cm-1 at room temperature (27°C), a value twice as high as that of an existing lithium ion conductor, Li3N (6 mS cm-1) and also exceeding even the ionic conductivity of organic electrolytes*2, which are used in conventional lithium-ion secondary batteries. Additionally, the crystal structure of Li10GeP2S12 was elucidated using the high resolution, high intensity neutron diffractometer (SuperHRPD) at the Japan Proton Accelerator Research Complex (J-PARC)*3, and a superionic conduction pathway*4 in the three-dimensional framework structure of Li10GeP2S12 was clarified. This discovery indicates that all-solid-state ceramic batteries*5 with properties of noncombustibility and high safety are promising candidates for next generation high energy density batteries. Development of such batteries is a fiercely competitive area of research as they represent key devices for the success of electric and hybrid vehicles and smart grids, thereby providing a new guiding principle in battery development. Results of the present study were published in Nature Materials on July 31, 2011(UK time).
In the present study, this research team has discovered a new sulfide material, Li10GeP2S12, which is a superionic conductor with the following properties: 1) noticeably high lithium-ion conductivity, 12 mS cm-1 at room temperature (27°C); 2) decomposition voltage exceeding 5 V (Fig. 1); and 3) functionality as an electrolyte material of all-solid-state batteries. In particular, this new material has superior ionic conductivity far exceeding that of organic electrolytes (Fig. 2). Moreover, precision neutron crystallographic analyses performed using the high resolution bank of the high resolution, high intensity neutron diffractometer, SuperHRPD (BL08), at the Japan Proton Accelerator Research Complex (J-PARC) revealed that Li10GeP2S12 has a unique three-dimensional structure (Fig. 3) in which lithium forms a series of chain-like units in its framework, yielding high lithium-ion conductivity to this material.
Batteries are key electricity-storing devices that are critical to enabling the widespread introduction of electric vehicles, plug-in hybrid vehicles, and smart grids into society. Developing the next generation of batteries that is superior to the existing lithium-ion batteries in terms of capacity, cost, and safety has become an urgent task. Electrolytes represent the key to developing batteries with these desirable properties. Existing lithium-ion batteries require safety devices because they involve the use of combustible organic electrolytes. Batteries composed of ceramic material alone are regarded as devices that would have ultimately superior stability; such batteries would be safe, reliable, superior, and long-lasting devices with higher capacity and higher output. However, properties of solid electrolytes prevent the practical utilization of ceramic batteries. In particular, the ionic conductivity of existing solid electrolytes is approximately 0.1 to 1 mS cm-1, a value that is one or more order of magnitude lower than that of organic electrolytes.
Materials and Methods
Examination of sulfide systems that were expected to have ionic conductivity as high as a superionic conductor led to the discovery of the superionic conductor Li10GeP2S12, which exhibited high ionic conductivity in the course of searching for new materials. The crystal structure of Li10GeP2S12 was determined by neutron diffractometry using the high resolution, high intensity neutron diffractometer SuperHRPD (BL08) at J-PARC. Moreover, the team revealed that batteries equipped with LiCoO2, which is commonly used as a cathode of lithium-ion batteries, exhibit superior properties, indicating the applicability of Li10GeP2S12 as a material.
The solid electrolyte material (Li10GeP2S12) that was discovered in the present study is applicable to making lithium batteries composed of solid-state material only, allowing the development of high capacity batteries with improved safety on the path to developing all-solid-state batteries as well as ultra-compact ceramic batteries. The present discovery will further accelerate the efforts in view of a prospect that next generation batteries are aiming at all-solid-state*6.EBy making existing lithium-ion batteries entirely solid state, new batteries superior in safety and stability with a long life can be developed, contributing to achieving even higher capacity. This research team is also pursuing the development of safer, more stable, and longer lasting batteries. Moreover, achieving higher safety, stability, and longer life is the greatest challenge in the development of next generation batteries (innovative batteries*7) that far exceed the capacity of existing batteries. An important step in this process is to replace combustible electrolytes with noncombustible or flame-retarding electrolytes. Successful development of noncombustible inorganic solid electrolytes exhibiting high ionic conductivity comparable or superior to that of organic electrolytes can significantly contribute to the realization of large-scale and high capacity batteries. This research group plans to pursue research toward high energy density batteries by further improving the conductivity and stability of the discovered material.
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